1. Field of the Invention
The present invention relates to an image forming apparatus, such as a copying machine, a laser beam printer, a laser facsimile machine, and a multifunction machine thereof, by which an image is formed on a sheet material.
2. Description of the Related Art
A number of image forming apparatuses such as a color copying machine and a color laser beam printer, by which an image is formed according to an electrophotographic type or an electrostatic recording type, adopt an intermediate transfer type in which an image forming apparatus has a configuration provided with a photosensitive drum and an intermediate transfer belt as an image bearing member. According to the intermediate transfer type, an image born on a photosensitive drum is transferred (primary transfer) onto the surface of an intermediate transfer belt, and the images on the intermediate transfer belt are transferred at a time onto a sheet material as a recording medium (secondary transfer), as disclosed in, for example, Japanese Patent Application Laid-Open No. 2002-244449. A number of image forming apparatuses adopting the intermediate transfer belt type have been proposed.
FIG. 8 shows a structure example of a secondary transfer portion including an intermediate transfer belt 3, and the like. Images formed on a photosensitive drum (not shown) as an image bearing member are transferred onto the intermediate transfer belt 3. The images transferred on the intermediate transfer belt 3 are transferred at a time onto a sheet of recording paper (hereinafter, called a sheet material S as a material onto which an image is transferred) conveyed from between resistration rollers 7 and 8. Immediately after the sheet material S is fed out through the pair of resistration rollers 7 and 8, the tip of the sheet material S is guided along both of side walls 1 and 2 (hereinafter, called secondary transfer feeding guides 1 and 2), which form a guide path (guide passage), toward between a pair of rollers 4 and 5 in the secondary transfer portion. The secondary transfer roller 4 as one of the roller pair forms one of a plurality of rollers which tightly stretch the intermediate transfer belt 3 for winding.
In the secondary transfer portion, the sheet material S is guided along the guide path formed with the secondary transfer feeding guides 1 and 2, and the images on the intermediate transfer belt 3 are transferred at a time onto the sheet material S, while the sheet material S is being pressed between the secondary transfer rollers 4 and 5, when the sheet material S is fed out through the pair of resistration rollers 7 and 8 which rotate by rotating power received from a motor (not shown) as a driving source. In this case, the resistration roller 8 as one of the roller pair, together with a roller bearing 9, is energized with a pressing spring 11, and is pressed against the resistration roller 7 as the other of the roller pair, and the secondary transfer roller 5 as one of the roller pair, together with a bearing arm 6, is energized with a pressing spring 10, and is pressed against the secondary transfer roller 4 as the other of the roller pair.
FIG. 9A and FIG. 9B are a schematic view of the behavior of the sheet material S just before the sheet material S is fed to between the secondary transfer rollers 4 and 5. The tip of the sheet material S fed out from between the resistration rollers 7 and 8 is guided as shown in FIG. 9A while being abutted against the secondary transfer feeding guide 2 in one of sidewalls, and is bent by being abutted against an abutment portion A (hereinafter, called an abutment point) which is a turning portion of the secondary transfer feeding guide 1 of the other sidewall. Subsequently, the sheet material S, first from the tip, enters into the secondary transfer portion of the pair of the secondary transfer rollers 4 and 5, as shown in FIG. 9B, for secondary transfer of the images on the intermediate transfer belt 3.
Following FIG. 9A and FIG. 9B, FIG. 10A and FIG. 10B also are a schematic view of the behavior of the sheet material S during being conveyed. When the rotational speeds V1 of the secondary transfer rollers 4 and 5 are larger and faster than the rotational speeds V2 of the resistration rollers 7 and 8, that is, in the case of V1>V2, the sheet material S is guided along the secondary transfer feeding guide 1 while being abutted thereagainst as shown in FIG, 10A. Conversely, in the case of V1<V2, the sheet material S is guided along the secondary transfer feeding guide 2 while being abutted thereagainst as shown in FIG. 10B.
Incidentally, the behavior of the sheet material S just before the sheet material S is fed into the secondary transfer portion has been shown as a general example in FIG. 8 through FIG. 10B. But the secondary transfer portion in the above embodiments has had the following structural problems which should be solved.
As shown in FIG. 8, the secondary transfer feeding guide 1 is integrally molded into a part of a conveying path frame 12. There are some cases in which, as shown in FIG. 11 of the above-described secondary transfer feeding guide 1 seen from above, a displacement is caused in the positioning of the conveying path frame 12 as a base, and the secondary transfer feeding guide 1 is positioned not parallel to the direction of the sheet width intersecting perpendicularly to the conveying direction of the sheet material S, but inclined at an angle α to cause a difference in the conveying direction, that is, a front and back difference (hereinafter, “front and back” is expressed as “front and far-side”) between both the ends 1a and 1b of the guide. The above front and far-side difference of the guide causes an abnormality, such as a displacement, of an image for which secondary transfer onto the sheet material S is executed.
Furthermore, the conveying path frame 12 is set with the maximum dimensional tolerance between the front and the far-side at positioning in some cases. Then, there is caused a front and far-side difference between the both sides, that is, for the width of a guide path in a conveying segment between the resistration rollers 7 and 8, and the secondary transfer roller 4 and 5. Thereby, timing at which the sheet material S enters into a nip portion between the secondary transfer rollers 4 and 5 is different from each other at the both sides of the tip of the sheet material S, and there is caused a phenomenon in which an image is transferred in a state in which the image is inclined in the front and far-side direction relative to the sheet material S. Accordingly, image magnifications are different from each other between the front and the far-side to cause an abnormality in an image.